U.S. patent application number 12/209350 was filed with the patent office on 2009-03-19 for process for the production of graphite electrodes for electrolytic processes.
This patent application is currently assigned to Bayer MaterialScience AG. Invention is credited to Jurgen Kintrup, Gerhard Moormann, Odo Muddemann, Frank Rauscher, Rainer Weber, Matthias Weis.
Application Number | 20090074954 12/209350 |
Document ID | / |
Family ID | 40104746 |
Filed Date | 2009-03-19 |
United States Patent
Application |
20090074954 |
Kind Code |
A1 |
Weber; Rainer ; et
al. |
March 19, 2009 |
PROCESS FOR THE PRODUCTION OF GRAPHITE ELECTRODES FOR ELECTROLYTIC
PROCESSES
Abstract
A process is described for the production of graphite electrodes
coated predominantly with noble metal for electrolytic processes,
especially for the electrolysis of hydrochloric acid, wherein the
surface of a graphite electrode is coated with an aqueous solution
of a noble metal compound and then tempered at 150 to 650.degree.
C. in the presence of reducing and/or extensively oxygen-free
gases.
Inventors: |
Weber; Rainer; (Odenthal,
DE) ; Kintrup; Jurgen; (Emsdetten, DE) ; Weis;
Matthias; (Leverkusen, DE) ; Muddemann; Odo;
(Koln, DE) ; Moormann; Gerhard; (Brunsbuttel,
DE) ; Rauscher; Frank; (Koln, DE) |
Correspondence
Address: |
CONNOLLY BOVE LODGE & HUTZ, LLP
P O BOX 2207
WILMINGTON
DE
19899
US
|
Assignee: |
Bayer MaterialScience AG
Leverkusen
DE
|
Family ID: |
40104746 |
Appl. No.: |
12/209350 |
Filed: |
September 12, 2008 |
Current U.S.
Class: |
427/77 |
Current CPC
Class: |
C25B 11/043 20210101;
C25B 11/097 20210101; C25B 11/081 20210101; C25B 1/26 20130101;
Y02E 60/36 20130101 |
Class at
Publication: |
427/77 |
International
Class: |
B05D 5/12 20060101
B05D005/12 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 15, 2007 |
DE |
102007044171.3 |
Claims
1. A process for producing graphite electrodes coated predominantly
with noble metal for electrolytic processes comprising (1) coating
the surface of a graphite electrode with an aqueous solution of a
noble metal compound, (2) removing the solvent, and (3) tempering
the graphite electrode at 150 to 650.degree. C. in the presence of
reducing and/or extensively oxygen-free gases.
2. The process of claim 1, wherein said electrolysis process is the
electrolysis of hydrochloric acid.
3. The process of claim 1, wherein said noble metal compound is at
least one compound selected from the group comprising iridium,
ruthenium, rhodium, platinum, and palladium compounds, or mixtures
thereof.
4. The process of claim 3, wherein said nobel metal compound is a
salt of an inorganic or organic acid or a complex compound.
5. The process of claim 4, wherein said noble metal compound is an
iridium, ruthenium, rhodium, platinum, or palladium halide,
acetate, oxalate, nitrate, or pentanedionate.
6. The process of claim 5, wherein said noble metal compound is an
iridium, ruthenium, rhodium, platinum, or palladium halide.
7. The process of claim 6, wherein said noble metal compound is an
iridium, ruthenium, rhodium, platinum, or palladium chloride.
8. The process of claim 7, wherein said noble metal compound is
iridium chloride.
9. The process of claim 8, wherein said iridium chloride is
IrCl.sub.3, IrCl.sub.4, or a mixture of IrCl.sub.3 and
IrCl.sub.4.
10. The process of claim 1, wherein the noble metal coating
produced contains 5 to 40 g/m.sup.2 of noble metal, based on the
area of the graphite electrode.
11. The process of claim 10, wherein the noble metal coating
produced contains 7.5 to 20 g/m.sup.2 of noble metal, based on the
area of the graphite electrode.
12. The process of claim 1, wherein said tempering takes place at
200 to 450.degree. C.
13. The process of claim 12, wherein said tempering takes place at
250 to 350.degree. C.
14. The process of claim 1, wherein said reducing and/or
extensively oxygen-free gases consist of a gaseous mixture of a
chemically inert gas.
15. The process of claim 14, wherein said gaseous mixture of a
chemically inert gas is a mixture of nitrogen or a noble gas, with
hydrogen.
16. The process of claim 15, wherein the proportion of hydrogen in
said mixture ranges from 1 to 5.5 volume %.
17. The process of claim 1, wherein the treatment time of said
tempering is 1 to 5 hours.
18. The process of claim 17, wherein said treatment time is 2 to 3
hours.
19. The process of claim 1, wherein the proportion of oxygen in
said reducing and/or extensively oxygen-free gas is at most 5
volume %.
20. The process of claim 19, wherein the proportion of oxygen in
said reducing and/or extensively oxygen-free gas is at most 3
volume %.
21. The process of claim 20, wherein the proportion of oxygen in
said reducing and/or extensively oxygen-free gas is at most 1
volume %.
22. A graphite electrode prepared according to the process of claim
1.
Description
RELATED APPLICATIONS
[0001] This application claims benefit to German Patent Application
No. 10 2007 044 171.3, filed Sep. 15, 2007, which is incorporated
herein by reference in its entirety for all useful purposes.
BACKGROUND OF THE INVENTION
[0002] The invention relates to a process for the production of
graphite electrodes coated with finely divided iridium for
electrolytic processes, especially for the electrolysis of
hydrochloric acid.
[0003] A process for the electrolysis of hydrochloric acid is
described in Ullmanns Encyclopedia of Industrial Chemistry,
Chlorine 10.1 Electrolysis of Hydrochloric Acid, 2006, Wiley-VCH
Verlag. The electrolysers typically used for the electrolysis of
hydrochloric acid consist of bipolar-connected graphite electrode
plates arranged in series according to the filter press principle.
Anode and cathode chambers are normally separated by a diaphragm or
a cation exchange membrane. Conventionally, chlorine is produced on
the anode side and hydrogen on the cathode side. Noble metal salts,
e.g. platinum, palladium and rhodium salts, are added continuously
or batchwise to the cathode chambers of the electrolysers in order
to lower the hydrogen deposition voltage and hence the cell
voltage, metallic noble metal being deposited on the graphite
electrodes. One substantial disadvantage of this procedure is that
the deposition of noble metal only produces the desired voltage
lowering effect for a short time and therefore has to be constantly
renewed, resulting, inter alia, in a high consumption of noble
metal. According to EP 683 247 A1, another disadvantage is that
noble metals can be deposited in the entire apparatus system
downstream of the cells.
[0004] EP 683 247 A1 describes a process for the production of
graphite electrodes in which noble metal coatings, e.g. iridium
and/or rhodium coatings, are produced in the pores of the graphite
surface. The graphite electrodes according to EP 683 247 A1 are
produced by introducing, into the graphite, solutions of iridium
salts or rhodium salts, or mixtures of iridium salts or rhodium
salts with salts of the other platinum group metals, in monohydric
or polyhydric alcohols having 2 to 4 carbon atoms or in alcohol
mixtures. The surface of the graphite body impregnated with the
solution is then heated for 2 to 10 minutes at a temperature
between 200 and 450.degree. C., to a depth of up to about 1 mm,
with open gas flames, which are applied to the impregnated graphite
body, vertically from top to bottom, only when the whole of the
impregnated graphite body is situated below the gas flames.
EMBODIMENTS OF THE INVENTION
[0005] An embodiment of the present invention is a process for
producing graphite electrodes coated predominantly with noble metal
for electrolytic processes comprising (1) coating the surface of a
graphite electrode with an aqueous solution of a noble metal
compound, (2) removing the solvent, and (3) tempering the graphite
electrode at 150 to 650.degree. C. in the presence of reducing
and/or extensively oxygen-free gases.
[0006] Another embodiment of the present invention is the above
process, wherein said electrolysis process is the electrolysis of
hydrochloric acid.
[0007] Another embodiment of the present invention is the above
process, wherein said noble metal compound is at least one compound
selected from the group comprising iridium, ruthenium, rhodium,
platinum, and palladium compounds, or mixtures thereof.
[0008] Another embodiment of the present invention is the above
process, wherein said nobel metal compound is a salt of an
inorganic or organic acid or a complex compound.
[0009] Another embodiment of the present invention is the above
process, wherein said noble metal compound is an iridium,
ruthenium, rhodium, platinum, or palladium halide, acetate,
oxalate, nitrate, or pentanedionate.
[0010] Another embodiment of the present invention is the above
process, wherein said noble metal compound is an iridium,
ruthenium, rhodium, platinum, or palladium halide.
[0011] Another embodiment of the present invention is the above
process, wherein said noble metal compound is an iridium,
ruthenium, rhodium, platinum, or palladium chloride.
[0012] Another embodiment of the present invention is the above
process, wherein said noble metal compound is iridium chloride.
[0013] Another embodiment of the present invention is the above
process, wherein said iridium chloride is IrCl.sub.3, IrCl.sub.4,
or a mixture of IrCl.sub.3 and IrCl.sub.4.
[0014] Another embodiment of the present invention is the above
process, wherein the noble metal coating produced contains 5 to 40
g/m.sup.2 of noble metal, based on the area of the graphite
electrode.
[0015] Another embodiment of the present invention is the above
process, wherein the noble metal coating produced contains 7.5 to
20 g/m.sup.2 of noble metal, based on the area of the graphite
electrode.
[0016] Another embodiment of the present invention is the above
process, wherein said tempering takes place at 200 to 450.degree.
C.
[0017] Another embodiment of the present invention is the above
process, wherein said tempering takes place at 250 to 350.degree.
C.
[0018] Another embodiment of the present invention is the above
process, wherein said reducing and/or extensively oxygen-free gases
consist of a gaseous mixture of a chemically inert gas.
[0019] Another embodiment of the present invention is the above
process, wherein said gaseous mixture of a chemically inert gas is
a mixture of nitrogen or a noble gas, with hydrogen.
[0020] Another embodiment of the present invention is the above
process, wherein the proportion of hydrogen in said mixture ranges
from 1 to 5.5 volume %.
[0021] Another embodiment of the present invention is the above
process, wherein the treatment time of said tempering is 1 to 5
hours.
[0022] Another embodiment of the present invention is the above
process, wherein said treatment time is 2 to 3 hours.
[0023] Another embodiment of the present invention is the above
process, wherein the proportion of oxygen in said reducing and/or
extensively oxygen-free gas is at most 5 volume %.
[0024] Another embodiment of the present invention is the above
process, wherein the proportion of oxygen in said reducing and/or
extensively oxygen-free gas is at most 3 volume %.
[0025] Another embodiment of the present invention is the above
process, wherein the proportion of oxygen in said reducing and/or
extensively oxygen-free gas is at most 1 volume %.
[0026] Yet another embodiment of the present invention is a
graphite electrode prepared according to the above process.
DESCRIPTION OF THE INVENTION
[0027] This process produces a noble metal coating that is stable
for a certain time under the operating conditions of hydrochloric
acid electrolysis and does not have to be renewed.
[0028] Disadvantages of the process according to EP 683 247 A1 are
the fact that the lowering of the overvoltage at the electrodes
modified by this process is still not optimal in the electrolysis,
the use of alcoholic solvents, which can form explosive mixtures in
air and therefore demand special safety measures in this process
operating with open flames, and the fact that the temperature
control during heating is imprecise due to large temperature
differences between the gas flame used, the impregnated graphite
surface and the bulk of the graphite.
[0029] The object of the invention is to provide an improved
process for the production of graphite electrodes for electrolytic
processes which does not exhibit the aforementioned
disadvantages.
[0030] The invention provides a process for the production of
graphite electrodes coated predominantly with noble metal for
electrolytic processes, especially for the electrolysis of
hydrochloric acid, which is characterized in that the surface of
the graphite electrode is coated with an aqueous solution of a
noble metal compound, the solvent is removed and the graphite
electrode is then tempered at 150 to 650.degree. C. in the presence
of reducing and/or extensively oxygen-free gases.
[0031] In particular, the finished coating on the electrode
contains at least 95 wt. %, preferably at least 99 wt. %, of noble
metal.
[0032] The noble metal compound used consists in particular of at
least one compound from the group comprising iridium, ruthenium,
rhodium, platinum and palladium compounds, especially salts of
inorganic or organic acids or complex compounds, on its own or in
any desired mixture. It is preferable to use iridium, ruthenium,
rhodium, platinum or palladium halides, acetates, oxalates,
nitrates or pentanedionates, and particularly preferable to use
halides of said noble metals, especially noble metal chlorides. It
is particularly preferable to use an iridium chloride, which can be
e.g. IrCl.sub.3 or IrCl.sub.4 or a mixture of the two. As water is
used as solvent, said compounds can also contain water of
hydration. However, it is also possible, for example, to use an
acidic iridium halide solution, e.g. hexachloroiridic(IV) acid.
[0033] The aqueous solution of noble metal compounds can
additionally contain surface-active substances, especially
surfactants, other salts or, in particular, mineral acids, and also
water-miscible organic solvents, especially alcohols or
ketones.
[0034] The amount of noble metal compound is preferably
proportioned so that the coating produced contains 5 to 40
g/m.sup.2, preferably 7.5 to 20 g/m.sup.2, of noble metal, based on
the area of the graphite electrode, i.e. the geometric surface area
defined by the external dimensions (edge lengths).
[0035] In one preferred variant of the process according to the
invention, the treatment in the reducing and/or extensively
oxygen-free gas atmosphere takes place at 200 to 450.degree. C.,
particularly preferably at 250 to 350.degree. C.
[0036] The treatment takes place in particular in an oven or
heating cabinet with the gases flowing over the coated surface of
the electrode. For this purpose the oven or heating cabinet has
e.g. a gas inlet orifice and a gas outlet and is sealed against the
admission of air from outside. For example, if the oven is not
completely gastight, its interior chamber can be operated at a
slightly higher pressure than the surrounding atmospheric air in
order to prevent air from entering. In particular, the treatment is
carried out with a residual air concentration of at most 25 vol. %,
preferably of at most 5 vol. % and particularly preferably of at
most 2 vol. %. The proportion of oxygen in the tempering gas is
particularly at most 5 vol. %, preferably at most 3 vol. % and
particularly preferably at most 1 vol. %.
[0037] Preferably, the gas atmosphere used consists of an inert
gas, especially nitrogen or a noble gas, preferably helium, argon,
neon, krypton, radon or xenon, or carbon dioxide, or a gaseous
mixture of one of said inert gases with hydrogen, or pure hydrogen.
The proportion of hydrogen can thus range from 0 vol. % (pure inert
gas) to 100 vol. % (pure hydrogen), but it is preferable to use a
hydrogen concentration ranging from 1 to 5.5 vol. %. The inert gas
used is particularly preferably nitrogen. Hydrogen/nitrogen
mixtures that are suitable in principle are commercially available
in ready-mixed form under the name of forming gas.
[0038] The treatment time in the reducing and/or extensively
oxygen-free gas atmosphere is preferably 1 to 5 hours and
particularly preferably 2 to 3 hours.
[0039] In one preferred embodiment of the invention, after the oven
has been loaded with one or more graphite electrodes, it is closed
and initially flushed at room temperature with the above-described
gas atmosphere until the residual air concentration is below 25
vol. %, preferably below 5 vol. % and particularly preferably below
1 vol. %. The oven is then heated to the target temperature and
left at this temperature for the chosen treatment time, while still
being flushed with gas during both these operations. The oven
chamber is then left to cool, while still being flushed with gas,
and the contents are removed once the temperature has fallen below
100.degree. C., preferably below 50.degree. C.
[0040] The invention also provides graphite electrodes coated with
noble metal which are obtained by the novel coating process.
[0041] The graphite electrodes coated by the process according to
the invention are outstandingly suitable for the production of
chlorine and hydrogen by the electrolysis of hydrochloric acid.
[0042] The invention therefore also provides the use of graphite
electrodes coated with noble metal, obtained by the novel coating
process, as electrodes (cathodes and/or anodes) in the production
of chlorine and hydrogen by the electrolysis of hydrochloric
acid.
[0043] The HCl concentration in the electrolysis of hydrochloric
acid with the graphite electrodes coated according to the invention
can be 5 to 36 wt. %. The hydrochloric acid used normally has an
HCl concentration of between 10 and 30 wt. %. The HCl concentration
is preferably in the range from 15 to 25 wt. %.
[0044] The electrolysis of hydrochloric acid with the graphite
electrodes coated according to the invention is conventionally
operated at a temperature of 30 to 100.degree. C., preferably of 50
to 100.degree. C. and particularly preferably of 70 to 90.degree.
C.
[0045] The graphite electrodes coated according to the invention
are preferably produced using electrode graphite (graphite for
technical electrolytic processes), e.g. a grade of graphite such as
AX from Graphite COVA GmbH, Rothenbach, or HL, ML or AL graphite
marketed by SGL Carbon GmbH, Meitingen. Such particularly suitable
types of graphite usually have a characteristic porosity
(cumulative pore volume) of 12 to 23%, the resistivity is 5.0 to
12.5 .mu..OMEGA.m, the bulk density is 1.60 to 1.30 g/cm.sup.3 and
the ash content is below 0.1%.
[0046] To improve the discharge of the gases formed in the
electrolysis (anode: chlorine, cathode: hydrogen), the surface of
the graphite electrodes can be structured e.g. by the introduction
of 1 mm to 3 mm wide slits 10 to 30 mm deep, spaced 3 to 7 mm
apart. The novel coating process is found to be particularly
advantageous in the case of graphite electrodes with a structured
surface because of the greater uniformity of the coating.
[0047] The diaphragms preferably used to separate anode and cathode
chambers in diaphragm electrolysis are preferably made of PVC
fabric, mixed PVC/PVDF fabric or PVDF fabric.
[0048] Membranes made of polyfluorosulfonic acids (e.g. Nafion.RTM.
430 membranes from DuPont) can also be used as an alternative.
[0049] The hydrochloric acid that is preferably to be used in
electrolysis with the graphite electrodes coated according to the
invention is obtained e.g. in the synthesis of organic compounds
such as polyisocyanates. It has proved advantageous to remove
impurities, especially organic impurities, from the hydrochloric
acid before it enters the electrolysis cells. This is done by
treating the hydrochloric acid with activated charcoal.
Alternatively, it can be also be treated with ozone or extractants.
Inorganic impurities can be removed by ion exchange methods.
[0050] The invention is illustrated in greater detail below with
the aid of the following Examples.
[0051] All the references described above are incorporated by
reference in their entireties for all useful purposes.
[0052] While there is shown and described certain specific
structures embodying the invention, it will be manifest to those
skilled in the art that various modifications and rearrangements of
the parts may be made without departing from the spirit and scope
of the underlying inventive concept and that the same is not
limited to the particular forms herein shown and described.
EXAMPLES
Example 1
Comparative Example
[0053] Hydrochloric acid was electrolysed in an electrolysis cell
having a PVC diaphragm and two uncoated graphite electrodes (AX-20
from COVA), each of which had an area of 100 mm.times.100 mm, a
thickness of 60 mm and fourteen 5 mm wide lands structured by means
of 13 slits approx. 2 mm wide and 19 mm deep. The hydrochloric acid
was pumped round an internal circuit at a rate of 6 l/h in both
electrode chambers. The distance between the surfaces of the
cathode and anode (both vertical) was 5 mm and the slits were in
the vertical direction. The cell housing was made of acid-resistant
and chlorine-resistant plastic. The cathode and anode were sealed
into the cell housing with current supply pins. The two halves of
the cell were separated by a PVC diaphragm. The electrolyte could
be pumped round both halves of the cell, the throughput being
varied in the range between 2 l/h and 10 l/h. Fresh 30%
hydrochloric acid was introduced into these circuits by means of
metering pumps in such a way that the subsequent concentration of
hydrochloric acid in the electrolyte chambers during electrolysis
was about 20 wt. %. The product gases and the impoverished
electrolytes leave the cell via gas/liquid separators. A current of
50 A, i.e. a current density of 5 kA/m.sup.2, was established by
means of an electrical power generator. The resultant cell voltage
was measured at the front edges of the electrodes with two graphite
tips, each insulated in the feed.
[0054] After a running-in period of 5 days, the cell voltage was
1.97 volt at a temperature of 75.degree. C.
[0055] The PVC diaphragm was then exchanged for a Nafion.RTM. 430
cation exchange membrane from DuPont. After a running-in period of
7 days, the cell voltage was 1.99 volt at a temperature of
81.degree. C.
Example 2
Comparative Example
[0056] 0.286 g of iridium(IV) chloride hydrate
(IrCl.sub.4.H.sub.2O, Ir content 52.23 wt. %) was dissolved in
1.245 ml of 1,2-ethanediol. Using a paintbrush, all of this
solution was uniformly applied to the 14 land surfaces (5
mm.times.100 mm each) of a graphite electrode having the same
structure and size as in Example 1. The amount of iridium applied
was 15.0 g/m.sup.2, based on the geometric area of the graphite
electrode (100 mm.times.100 mm). After approx. 15 minutes the side
treated with the solution (subsequently the cathode side in the
electrolysis) was heated for 5 minutes with a flame from a
butane/propane gas burner, a temperature of 450.degree. C. being
reached after 5 minutes and the plate already being situated below
the burner before the flame was ignited. After cooling to below
90.degree. C., the land surfaces of the graphite electrode were
uniformly coated with 1.245 ml of 1,2-ethanediol (without addition
of metal salt) and the heating was then repeated immediately
(without a waiting time). The graphite plate was built as the
cathode into the electrolysis cell described in Example 1. With
electrolyte throughputs of 6 l/h and using a PVC diaphragm, the
resultant cell voltage, which remained constant for 8 days, was
1.77 volt at a current density of 5 kA/m.sup.2 and a temperature of
75.degree. C.
Example 3
Inventive Example
[0057] 0.289 g of iridium(IV) chloride hydrate
(IrCl.sub.4.H.sub.2O, Ir content 52.23 wt. %) was dissolved in
1.512 g of deionized water. Using a paintbrush, all of the solution
was applied to the 14 land surfaces (5 mm.times.100 mm each) of a
graphite electrode having the same structure and size as in Example
1 to give an iridium loading of 15.0 g/m.sup.2, based on the area
of the graphite electrode (100 mm.times.100 mm). The coated
electrode block was then immediately treated in a vertical tube
oven having an internal diameter of 15 cm and an internal volume of
approx. 5 l, the electrode block initially being flushed for a
period of 30 minutes at room temperature with a gaseous mixture
consisting of 5 vol. % of hydrogen and 95 vol. % of nitrogen at a
volumetric flow rate of 50 l/h. The oven was then heated to
250.degree. C. at a rate of approx. 10.degree. C./minute and the
electrode block was tempered for a period of 3 h with the gas still
flowing. The oven heating was then switched off and the electrode
block was cooled with the gas still flowing. After approx. 3 hours
the oven temperature had cooled to below 100.degree. C., the gas
flow was switched off and the closed oven cooled further overnight
to a temperature below 50.degree. C.; only then was it opened to
remove the electrode.
[0058] The finished graphite electrode was built as the cathode
into the electrolysis cell described in Example 1. With an
electrolyte throughput of 6 l/h and using a PVC diaphragm, the
resultant cell voltage on the fifth day of operation was 1.59 volt
at a current density of 5 kA/m.sup.2 and a temperature of
75.degree. C. The experiment was continued for a period of up to
150 days with cut-offs and variations in the current density and
temperature, but there was no detectable loss of quality.
Example 4
Inventive Example
[0059] 0.289 g of iridium(IV) chloride hydrate
(IrCl.sub.4.H.sub.2O, Ir content 52.23 wt. %) was dissolved in
1.525 g of deionized water and applied to the land surfaces of a
graphite electrode as in Example 3. The subsequent treatment in the
oven was also carried out as in Example 3, the only difference
being that the oven was heated to a temperature of 450.degree. C.
and the treatment time at this temperature was 2 h.
[0060] The finished graphite electrode was built as the cathode
into the electrolysis cell described in Example 1. With an
electrolyte throughput of 6 l/h and using a PVC diaphragm, the
resultant cell voltage on the eighth day of operation was 1.73 volt
at a current density of 5 kA/m.sup.2 and a temperature of
74.degree. C. The experiment was continued for a period of up to 45
days with cut-offs and variations in the temperature, but there was
no detectable loss of quality.
Example 5
Inventive Example
[0061] 0.190 g of ruthenium(III) chloride hydrate
(RuCl.sub.3.H.sub.2O, Ru content 40.07 wt. %) and 0.143 g of
iridium(IV) chloride hydrate (IrCl.sub.4.H.sub.2O, Ir content 52.23
wt. %) were dissolved in 1.504 g of deionized water. Using a
paintbrush, all of the solution was applied to the 14 land surfaces
(5 mm.times.100 mm each) of a graphite electrode having the same
structure and size as in Example 1 to give a ruthenium loading of
7.6 g/m.sup.2 and an iridium loading of 7.5 g/m.sup.2, based on the
area of the graphite electrode (100 mm.times.100 mm).
[0062] The oven treatment was carried out analogously to Example
3.
[0063] The finished graphite electrode was built as the cathode
into the electrolysis cell described in Example 1. With an
electrolyte throughput of 6 l/h and using a Nafion.RTM. 430 cation
exchange membrane, the resultant cell voltage on the fifth day of
operation was 1.66 volt at a current density of 5 kA/m.sup.2 and a
temperature of 67.degree. C.
* * * * *